Fuel pumps are used in motor vehicles to transfer liquid fuel, typically gasoline or diesel from a fuel tank to an internal combustion engine. The pump is driven by a small DC motor and to minimize fuel leakage through bearing seals etc, the fuel passes through the interior of the motor. This works very well even with motors having commutators, with the fuel cooling the motor and eliminating sparking between the brushes and the commutator. However, with the advent of high alcohol fuels, chemical reactions between the commutator and the fuel has become problematic leading to the use of graphite commutators and renewed interest in brushless motors to drive the fuel pumps. There are many advantages of brushless motors, especially in automobile applications, such as longer life by eliminating the use of brushes and a commutator.
One problem with the use of BLDC motors in fuel pumps is that the fuel has a very restricted pathway through the motor which causes a severe restriction to the free flow of fuel. One reason for this is that BLDC motors have a wound stator and due to the aggressive nature of the fuel it is desirable to protect the stator windings. This is usually done by over moulding the stator, core and windings, with over mould material such as a plastics material or a resinous material, preferably using an insert moulding technique. This technique, unfortunately, transforms the stator into a solid mass, closing off the various gaps between the stator core and the windings. As the stator core is usually pressed into the pump housing, the only remaining pathway for the fuel is through the small gap between the stator and the rotor. However, this gap is intentionally made as small as possible to increase the efficiency of the motor. Fuel in this small gap is caught between the rotating rotor on one side and the stationary stator on the other side causing frictional heating of the fuel as well as causing considerable drag on the rotor, resulting in a significant lowering of the motor efficiency. This problem does not exist in the PMDC motors having a stator formed with segment magnets due to the channels existing between the individual magnets.
The term brushless direct current motor is used in this specification is used in its widest sense and is intended to include those special BLDC motors known as BLAC motors which have a similar physical structure but are designed to operate with sinusoidal power signals from the motor controller.